1. Field of the Invention
The present invention relates to electric induction motors. More particularly, the invention relates to electric induction motors and related methods of cooling. While the invention is subject to a wide range of applications, it is especially suitable for use in an electric vehicle propulsion system; and will be particularly described in that connection.
2. Description of the Related Art
Conventional electric motors, and, in particular, squirrel cage induction motors, generally consist of a stator and a rotor. Both the stator and rotor are comprised of a core of magnetic laminations containing conductors typically made of copper. The stator also includes conductive end turns at each end of its core, while the rotor includes conductive end caps at its ends.
Operation of the motor generates heat in the core and conductors of both the stator and rotor. In order to avoid overheating and failure of the motor, the stator and rotor must be cooled. According to one conventional cooling technique, a coolant, typically oil, is pumped into the motor housing. The coolant is directed to an array of spray nozzles located at each end of the motor housing. The nozzles spray the coolant at the conductive end turns of the stator and the end caps of the rotor. The coolant then drains by gravity into a motor sump, where it is pumped out of the motor, through an oil cooler, and then back to the spray nozzles.
According to this technique, the coolant contacts only the conductive end turns and end caps of the stator and rotor respectively. The conductors, especially those in the stator core, are the main sources of heat within the motor. Heat generated in these conductors must travel by conduction from the center to the ends of the stator. At the high power levels typically encountered in electric vehicle applications, this cooling technique may not sufficiently cool the motor, resulting in motor failure.